1. Field of the Invention
The present invention relates to a method of manufacturing a zinc oxide (ZnO) based (hereinafter, also referred to as ‘ZnO-based’) semiconductor device and, more particularly, to a ZnO-based compound semiconductor device having a contact electrode with high adhesion properties and good ohmic contact characteristics, as well as a method of manufacturing the same.
2. Description of the Related Art
ZnO is a direct transition type semiconductor having a band-gap energy of 3.37 eV at room temperature and is expected to be used as a material for a photoelectric device of a wavelength range from blue to ultraviolet. In particular, ZnO has the physical properties extremely suitable for a semiconductor light emitting device wherein an exciton binding energy is 60 meV and a refractive index is 2.0 (n=2.0). In addition, ZnO is not limitedly used in such light emitting diodes and/or light receiving diodes, but, may be employed in a wide range of devices including surface-acoustic wave (SAW) devices, piezoelectric devices, and the like. Moreover, ZnO as a raw material has the advantages that it is economical and is not harmful to the environment and human body.
It is well known that metal is poorly adhered to an oxide crystal and easily peels off or is easily separated therefrom. In particular, semiconductors not containing oxygen (for example, AlGaAs, InAlGaP, InAlGaP, InGaN, etc.) do not have significant problems in terms of attachment and/or adhesion to an electrode metal. However, a ZnO semiconductor, which is a metal oxide, exhibits poor adhesion to metal materials such as gold (Au), silver (Ag), rhodium (Rh), platinum (Pt), palladium (Pd), etc. Therefore, a process for manufacturing a p-electrode had a problem in that a metal electrode formed on a ZnO film often peels off or is separated therefrom. See, Japanese Laid-Open Patent Application No. 2003-110142 (hereinafter, also referred to as Patent Document 1) and Japanese Laid-Open Patent Application No. 2004-207440 (hereinafter, also referred to as Patent Document 2).
Furthermore, when heat treatment (e.g., alloying, sintering) is conducted after formation of an electrode in order to decrease contact resistance between a p-type ZnO layer and the electrode (i.e., to improve ohmic contact properties), there is a problem that peeling or separation of the electrode becomes pronounced. As such, with respect to an electrode of semiconductor device such as a semiconductor light emitting device, various manufacturing processes including, for example, heat treatment to decrease contact resistance, or die-bonding, wire-bonding, resin sealing, and the like entail heat stress and/or external stress. In addition, various types of stresses may be applied to the device after the manufacturing process. For instance, a device sealing process or a process of bonding the device to a circuit board may include application of heat stress thereto. The sealing process may also entail mechanical stress caused by sealing resin. Furthermore, heat or strain force may cause various stresses during use of a semiconductor device. For example, a semiconductor light emitting device used in an automobile may be subjected to various kinds of stresses including, heat and/or strain force, wherein such stresses are generated by car temperature, engine temperature, heat-shock due to diurnal and/or seasonal variation in temperature, exposure to solar UV radiation, car corrosion caused by water content and/or ambient gas (such as sulfate, chlorine, ozone), and so forth. Accordingly, it is very important to fabricate an electrode with excellent peeling-resistance capability independent of various stresses, thus ensuring high device performance, production yield and reliability.
Since ZnO-based compounds are wide band-gap semiconductors, metal materials having excellent ohmic characteristics which can be used as p-electrodes are limited. Therefore, there is still a strong demand for a metal electrode with high adhesion as well as excellent low-resistance ohmic-contact properties in order to provide manufacturing of improved ZnO-based semiconductor devices.